In recent years, as new type of secondary battery with high output and high energy density, a non-aqueous electrolyte secondary battery with light weight and high electromotive force has come to be used, which is based on a non-aqueous electrolyte containing lithium salt dissolved in a non-aqueous solvent and which effects charge and discharge, making use of oxidation and reduction of lithium.
In such a non-aqueous electrolyte secondary battery, lithium transition metal composite oxides such as lithium cobalt composite oxide, lithium nickel composite oxide and lithium manganese composite oxide have been widely used as cathode active material of the cathode. As anode active material of the anode, carbonaceous materials such as cokes, artificial graphite and natural graphite have been widely used either singly or as a mixture. As non-aqueous electrolyte, a non-aqueous solvent such as propylene carbonate or dimethyl carbonate, in which a lithium salt such as LiPF6 or LiBF4 is dissolved, has been used, for example.
However, in the non-aqueous electrolyte secondary battery mentioned above, the non-aqueous solvent in the non-aqueous electrolyte tends to be decomposed as a result of reaction on the surface of the anode based on carbonaceous material, thus leading to deterioration of storage characteristics and cycle characteristics of the battery.
In this connection, it has previously been known that, by using ethylene carbonate as non-aqueous solvent in a non-aqueous electrolyte, the above-mentioned decomposition tends to be suppressed and a portion of the decomposition products produced form a desirable protective layer on the surface of the anode. Because of this, ethylene carbonate has been mainly used as non-aqueous solvent.
However, even when ethylene carbonate is mainly used as non-aqueous solvent, the non-aqueous solvent is gradually decomposed as a result of reaction on repeated charges and discharges, and deterioration of storage performance and cycling performance remained to be a problem.
Therefore, recently, a proposal has been made, by adding a small amount of protective layer-forming agent such as vinylene carbonate to a non-aqueous electrolyte, to form an effective protective layer on the surface of the anode based on carbonaceous material during initial charge and discharge, thus bringing about improvement in storage performance and cycle performance of the non-aqueous electrolyte secondary battery (refer to Patent Documents 1 to 3, for example).
On the other hand, in recent years, in order to enhance charge-discharge capacity per unit mass or unit volume of a non-aqueous electrolyte secondary battery, a proposal has been made to use, as anode active material in the anode, a metal such as tin or silicon or its oxide capable of intercalating and deintercalating lithium ions instead of carbonaceous material mentioned above (refer to Non-patent Document 1, for example).
As anode using this type of anode active material, a proposal has been made for the anode wherein a thin layer of anode active material such as silicon or tin is formed on the current collector by such methods as CVD, sputtering, vapor deposition, thermal spraying or plating. It has been shown that high charge-discharge capacity and excellent charge-discharge cycle performance can be obtained by using this kind of anode. Namely, in this kind of anode, the thin layer of the anode active material is divided into columns by gaps formed in the thickness direction of the layer and the bottom of these columns is in tight contact with the current collector. Through the gap formed around the columns, the stress, which is caused by expanding/shrinkage of the thin layer of the anode active material accompanying charge-discharge cycle, is released, and generation of the type of stress which works to disconnect the thin layer of the anode active material from the current collector is prevented, leading to excellent charge-discharge cycle performance (refer to Patent Document 4 and 5, for example).
[Patent Document 1] Japanese Patent Application Laid-Open Publication (Kokai) No. H6-52887
[Patent Document 2] Japanese Patent Application Laid-Open Publication (Kokai) No. H8-45545
[Patent Document 3] Japanese Patent Publication No. 3059832
[Patent Document 4] Japanese Patent Application Laid-Open Publication (Kokai) No. 2002-83594
[Patent Document 5] Japanese Patent Application Laid-Open Publication (Kokai) No. 2002-279972
[Non-Patent Document 1] Solid State Ionics. 113-115-57 (1998)